Please vote on this idea...
if we could get page layouts for service requests, then your Cell Phone SRD could go into a Cell Phone page layout that had the phone number field.
Every Service Request Definition can have its set of "Input" fields which can be configured under the Input tab. So for "Cell Phone" type SRD, you can add the Input field for phone number.
Additionally, within the SR itself, you can use the conditional display property on any Input field to be dependent on any other field's value: